mkinitrd

i successfully installed my rpm kernel as per instructions (Howtoforge,compile kernel the mandriva way) now im in the stage of creating a ramdisk for newly installed kernel, but when i excute the command mkinitrd /boot/initrd-2.6.31.5-default-1mnb.img 2.6.31.5-default, it gives a message saying "no modules available for kernel "2.6.31.5-default",